Three-quarters of people with COVID-19 test positive for antibodies, study finds

Feb. 12 (UPI) -- Blood tests accurately detect antibodies against COVID-19 in about 75% of people with confirmed cases of infection, but are less reliable in women and older adults, a study published Friday by JAMA Network Open found.

Antibody tests were slightly less accurate in women and older adults compared to men and younger adults, most likely due to lower levels of the cells, which are created by the immune system to fight off the virus, the researchers said.

Tests accurately detected antibodies in women who were positive for COVID-19 based on PCR screening 72% of the time, compared to 79% of the time in men, the data showed.

The tests were also accurate more than 80% of the time in adults age 40 to 59, but less than 75% percent of the time in those age 70 and older, according to the researchers.

That's potentially significant, given that older adults are at increased risk for serious illness from the virus, they said.

"We were only able to detect antibodies in a maximum of 75% to 80% of those tested, after their positive [COVID-19] PCR test," study co-author Dr. Atul Butte told UPI.

"We did not explore why this was under 100% [but] it could be a problem with our ability to measure the antibodies, a false-positive PCR test or an actual issue for those patients to make these ... antibodies," said Butte, director of the Bakar Computational Health Sciences Institute at the University of California, San Francisco.

Researchers have long known that older adults produce fewer antibodies than younger people, and several studies have observed gender differences as well, at least with COVID-19.

Although the differences are relatively small, they may help inform efforts to properly prioritize people for vaccination against the virus, according to Butte.

For this study, Butte and his colleagues performed antibody testing on nearly 500 adults who were confirmed positive for COVID-19 by the University of California Health System between Aug. 1 and Oct. 20 of last year.

Among those tested, 365, or 75%, were positive for virus antibodies, the researchers said.

Test accuracy was highest in men 126 days after they received a confirmed diagnosis of COVID-19 and in women 133 days after diagnosis, suggesting that antibody levels may be highest roughly four months after infection, the data showed.

"Many regions are already prioritizing the elderly for vaccination, and already immunizing individuals even if they already had COVID-19, [and] our data here would be consistent with those policies," Butte said.

"It will be important to run long-term studies to see how antibody responses wane over time, whether from having COVID-19 itself, or from vaccination, [but] it is also important to note that antibody responses are just one part of an immune response," he said.